In the past, the most common type of food container for frozen foods and other pre-prepared food products which are to be heated within the container were constructed of thin aluminum sheets and covered with an aluminum foil. While these aluminum trays are excellent for conventional oven cooking, they are costly and cannot be used in microwaves. Due to the increased popularity of microwave cooking, substantial efforts have been made to provide alternative containers which are suitable for both conventional oven and microwave oven cooking. A dual ovenable food tray has become an industrial objective.
Typically, dual ovenable food trays comprise a plastic-coated paperboard substrate. Although polyethylene and other common plastics have been used as a coating material due to their good moisture impermeability and good adherence to paperboard, such plastics tend to melt at high oven temperatures. For example, polyethylene terephthalate (PET) has found widespread popularity in coating compositions for paperboard substrates due to its high melting temperature and structural strength. However, food trays coated with PET-based coating compositions are generally limited for use in oven temperatures up to about 350.degree. F. At higher temperatures, PET-coated food trays tend to undergo severe deformation, e.g., curling, and lose their shape. Further, food tends to stick to PET-based coatings since such coatings generally exhibit poor food release properties.
Aqueous coating formulations have also been developed for coating paperboard substrates for use as food trays. For example, U.S. Pat. No. 4,418,119 to Morrow describes a paperboard or similar substrate coated first with a polyvinyl alcohol and then with a silicone. In U.S. Pat. No. 4,421,825 to Seiter, a two layer coating is provided, the first layer being an acrylic polymer comprising titanium dioxide and the second layer being a clear acrylic copolymer. U.S. Pat. No. 4,775,560 to Katsura teaches the use of an aqueous dispersion of an epoxy-acrylic resin/hiding pigment as a coating material for a heat resistant container.
Despite the many types of coating materials known in the prior art for preparing food trays, there still exists a need for a dual ovenable food tray which exhibits both high temperature resistance and superior food release properties. The coating material providing these properties should also be inexpensive to produce and apply, should with minimal modification find application in less demanding environments, such as frozen food cartons, liquid packaging, fresh foods and non-foods.